1. Field of the Invention
The present invention relates to methods and apparatus for measuring pollutants contaminating earth formations, more particularly, two and three-dimensional mapping of the resistivity of near surface volumes of earth formations.
2. Description of the Related Art
The pollution of near-surface earth formations, is a problem drawing worldwide concern. Leaking underground gasoline, fuel oil and other hydrocarbons storage tanks are a major source of pollution of potable ground waters. Likewise, brine used in borehole drilling operations and stored in surface and underground storage containers is also a major ground water pollutant. Another source of pollution is natural saline water from deep formations which often migrates toward shallow potable ground water formations, the initial path of migration being around an oil and gas well casing which has not been properly cemented to obtain the desired zonal isolation.
The detection, quantification and monitoring of pollutants near the surface of the earth has historically been an expensive and time consuming operation. Traditional techniques involve the drilling of a multiplicity of shallow boreholes throughout an area of concern, retrieving core samples from the drilling operation, and analyzing the core samples for pollutants. These techniques have not detected polluted areas which have not been penetrated by a borehole. Another technique involves the drilling of a multiplicity of monitor wells throughout the area of concern. Sensors are temporarily or permanently placed at varying depths within the monitor wells, and various parameters indicative of the pollutants are measured as a function of time. In this way, either the boreholes penetrate the pollutants or the pollutants will migrate with time to the vicinity of one or more boreholes thereby being detected by the borehole sensors. This technique is particularly expensive and time consuming if detection requires the pollutants to migrate to the vicinity of the borehole before being detected. There is a high probability that discrete sampling techniques such as well bore core sampling or even monitor well measurements over time will not detect pollutants until they have contaminated an extensive volume of earth formation. At this point, remedial clean up is extremely costly and significant environmental damage has already occurred.
Pollutants such as hydrocarbons and brine usually exhibit significantly higher resistivity and lower resistivity, respectively, than more virgin earth formations. In detecting such pollutants, surface transmitters and well borehole receivers have been used for determining the resistivity of intervening earth formation. U.S. Pat. No. 2,746,009 to McLaughlin et al discloses a method directed to locating ore bodies. Main and vernier transmitter coils are set at one location at the surface and a receiver is conveyed along a nearby borehole. The receiver coil detects primary signal and any secondary signal resulting from a low resistivity ore body anomaly. The vernier coil is rotated about its axis giving measurements which are related to the direction of an ore body in a plane comprising the main transmitter, the vernier transmitter, the downhole receiver, and the ore body. Similarly, Ruehle discloses in U.S. Pat. No. 3,391,334 the use of a surface transmitter and well borehole receivers, in particular, three vertically spaced borehole receivers to measure resistivity of a layer of earth formation bounded by the vertical extent of the receiver array. A three-dimensional subterranean map of resistivity cannot be obtained using the methods disclosed by McLaughlin et al and Ruehle.
U.S. Pat. Nos. 4,323,848 and 4,502,010 to Kuckes are directed to a means of measuring the resistivity of earth formations using a surface transmitter and a downhole receiver comprising a magnetometer. The thrust of both inventions is directed to improvements of the magnetometer receiver. Two embodiments of the surface transmitter are disclosed. The first embodiment comprises a circular antenna concentric with the top of the wellbore with the axis of the loop being perpendicular to the plane of the surface of the earth. The second embodiment comprises two electrodes inserted into the earth surface on opposite sides of the top of the borehole. In both embodiments, the transmitter remains fixed with respect to the position of the borehole. Therefore, spatially continuous, three-dimensional measurements of resistivity cannot be obtained using the means or methods taught by Kuckes.
U.S. Pat. Nos. 4,748,415; 4,901,023; 5,065,100; and 5,260,661 to Vail, III are directed toward measuring the resistivity of earth formation from within a borehole cased with highly conducting material such as steel. Surface transmitters are employed as well as downhole receivers. In one embodiment, two surface transmitters comprising circular antennas are used with the first being concentric with the top of the borehole and the second being positioned remote from the borehole with the axis being perpendicular to the plane of the earth's surface. The transmitters remain fixed with respect to the position of the borehole. Spatially continuous measurements, however, are not obtainable using the disclosures of Vail III.
A method of generating a spatially continuous, three-dimensional map of near-surface volumes of earth formation for detecting pollutants is that in a commonly owned U.S. Pat. No. 5,652,519, granted on Jul. 29, 1997. The surveying method disclosed in that patent application is directed to vertical induction mapping with measurements obtained in a borehole. A vertical induction survey requires one or more drill holes to be placed in or around the area to be surveyed. After a vertical induction survey is taken, the data collected is used in generating a vertical induction map to study and locate possible pollutant areas. The vertical induction data, however, was not susceptible to being independently verified with this technique. In addition, several boreholes might be made during a vertical induction survey in order to locate a borehole position suitably close to anomalous areas. However, the locations of the anomalies were unknown before vertical induction logs were taken.